U.S. Pat. No. 4,450,356 discloses remotely detecting gases in the atmosphere by using a frequency-mixed CO.sub.2 laser beam formed by passing the beam from a first CO.sub.2 laser through a frequency doubler and then frequency-adding the output to the frequency from a second CO.sub.2 laser to obtain wavelengths in the 3 micron region. A first wavelength in this region, preselected for nonabsorption by the gases to be detected, is transmitted through the gases toward an object capable of reflecting the beam back. A second wavelength preselected as highly absorbed by the gases to be detected is then transmitted. The presence and quantity of the gases is then determined by the difference in the amount respectively absorbed at the two wavelengths.
U.S. Pat. No. 4,490,043 discloses laser scanning apparatus for monitoring of gaseous pollutants (e.g. in a chemical plant) in which two laser beams having different wavelengths (one corresponding to an absorption line of the gas to be monitored) and modulated at different frequencies are combined into a single scanning beam. A portion of the scattered radiation is collected, detected and measured to give, for each chosen beam direction, the amount of the gas being monitored. The amount of radiation reaching the detector from the laser source is varied according to a predetermined program or in response to an external stimulus, and by this means the detector can be protected against severe overload when the beam scans over positions of abnormally high reflectivity.
U.S. Pat. No. 4,489,239 discloses a portable laser system for remote detection of methane gas leaks and concentrations. The system transmitter includes first and second lasers, tuned respectively to a wavelength coincident with a strong absorption line of methane and a reference wavelength which is weakly absorbed by methane gas. The lasers are aimed at a topographical target along a system axis and the beams successively interrupted by a chopper wheel. The system receiver includes a spherical mirror for collecting the reflected laser radiation and focusing the collected radiation through a narrowband optical filter onto an optial detector. The filter is tuned to the wavelength of the two lasers, and rejects background noise to substantially improve the signal-to-noise ratio of the detector. The output of the optical detector is processed by a lock-in detector synchronized to the chopper, and which measures the difference between the first wavelength signal and the reference wavelength signal.
In the 3.0 to 3.5 micron region there are atmospheric absorption features. Water is the dominant molecular absorber in this region with trace absorption due to carbon dioxide. Ideally one would like to get all of one's laser energy to the target area, i.e., 100% transmission. However, in the lower atmosphere at ranges greater than one km (kilometer), less than 100% of the transmitted energy makes it to one kilometer. The amount of loss that is acceptable is determined by the absorption characteristics of the selected species and the nature, or wavelength extent of the interfering species. Using methane and propane as the target species, these species have absorption features of 80% and 60% respectively. A loss of 50% of the laser energy at one km is not unacceptable. Therefore, there are two windows in the atmosphere between 3.0 and 3.5 microns.
The first is from 3.16 to 3.18 microns and may be suitable for methane detection. The second, is between 3.38 and 3.51 microns, where ethane, propane and butane can be detected. However, water vapor absorption is the dominate absorption feature and a DIAL lidar measures various laser lines for loss. Therefore, a change in the relative humidity will produce a loss in the various laser lines, which is not linear between these lines. But a water vapor correction can be made if the lidar measures water vapor along the beam path.